The invention relates to a circuit arrangement for operating a high pressure discharge lamp with a current having opposite polarities in successive periods, which lamp is provided with at least two main electrodes being spaced on an electrode distance from each other, the circuit arrangement comprising:
input terminals for connecting a supply source, PA1 output terminals for connecting the high pressure discharge lamp, and PA1 means, coupled to the input terminals, for supplying the lamp current to the high pressure discharge lamp, which lamp current, in the successive periods has a predetermined shape. PA1 means for detecting a first parameter indicative for the electrode distance and forming a first signal dependent on the first parameter, and with PA1 means for reshaping the lamp current in dependence on the thus formed first signal. PA1 means for detecting a second parameter indicative of the occurrence of lamp flicker and forming a second signal dependent on the detected second parameter, and PA1 means for further adjusting the shape of the lamp current in successive periods in dependence of the second signal.
Such a circuit arrangement is known from U.S. Pat. No. 5,608,294. The known circuit arrangement provides a measure to suppress flickering of a high pressure discharge lamp and is in particular suitable for operating a high pressure discharge lamp in a projection system like a projection television apparatus. In the known circuit arrangement, the lamp is supplied with successive block shaped current pulses of opposite polarity. The suppression of flickering is achieved by supplying, during periods of the lamp current, additional current pulses with the same polarity at the end of a predetermined fraction of such a period of the lamp current. By means of the thus reshaped current pulses, the temperature of the electrode is raised to a relatively high value, which high temperature increases the stability of the discharge arc, because the discharge arc originates from the same place on the electrode in each cathodic phase and so flickering is substantially suppressed. The additional current is supplied in a regular sequence, preferably during each successive pulse. Although it is known that AC operation of high pressure discharge lamps with a low frequency alternating lamp current prevents a rapid erosion of the electrodes of the high pressure discharge lamp (further also referred to as the lamp) and allows operation of the lamp with a relatively high efficacy, it has occurred that lamps operated with the known circuit arrangement showed to have a continuous increase of the arc voltage over an operating time of several hundred hours, which voltage increase appeared to continue when the lamp was experimentally operated for several thousand hours. As a luminous output of the lamp being fairly constant over the life of the lamp is of vital importance for use in a projection system, a continues arc voltage increase forms a serious drawback in reaching a long lamp life.
In case a high pressure discharge lamp is operated with an AC current, each electrode of the lamp alternately functions as a cathode and as an anode during successive periods of the lamp current. During these periods the electrode is said to be in the cathodic phase and the anodic phase, respectively. Electrode material, that is removed from the electrode in the anodic phase, returns to the electrode as a stream of ions in the cathodic phase. These transport processes further complicate the behavior of the electrode temperature during each period of the lamp current, since the time dependency of the electrode temperature in the anodic phase differs from that in the cathodic phase.